The present invention relates to processes for manufacturing composite parts, in particular in the case of parts having complex shapes. It relates more particularly to composite parts which are subject to very high mechanical stress.
One process for manufacturing composite parts consists in compression-molding a paste under high pressures, then effecting complete polymerization before demolding. The paste has been prepared beforehand, and comprises a mixture of resin and short fibers. This process is very widely used owing to its suitability for the manufacture of parts having complex shapes and to its high productivity. However, the compression-molding process is incompatible with the use of long reinforcement fibers. For this reason, it is not possible to consider producing parts which are mechanically very highly stressed by this process.
Other techniques are known which make it possible to use long reinforcement fibers. One of these techniques is referred to as “pultrusion”. It consists of unwinding the fibers of unlimited length, and immersing them in a resin bath to impregnate them. Then they are drawn through a heated die, then through a heated chamber where polymerization takes place. In this manner, it is possible continuously to draw products of any section, dictated by the shape of the die. However, these are always straight products. Another known technique is filament winding. Sets of pre-impregnated reinforcement fibers are wound on a mandrel which is mobile in rotation and in translation. The manufactured object is polymerized in an oven. Tubes, or parts of large dimensions, such as reservoirs, are obtained. But apart from the fact that the variety of forms is very limited, it is difficult to position the fibers entirely in the thickness of the manufactured wall. They have a tendency to move towards the surface of the mandrel. It is also difficult to maintain a constant proportion of fibers throughout the thickness of the wall.
Also known are techniques for molding parts made of composite material which use in particular a preform which itself is molded, to facilitate the positioning of the reinforcement fibers. It was proposed in European Patent 0 655'319 to stabilize a resin preform comprising reinforcement fibers by heating the preform to obtain a pasty consistency, the viscosity of which still permits compression-molding. Then the pasty reform is demolded and is placed in a second mold which is brought to a higher temperature, in order to effect compression-molding of the pasty preform while effecting polymerization.
However, the problem of such treatment is that it is difficult to control the stage at which what shall be named “a prepolymerization” is brought about (initial partial polymerization). Now, it is necessary to achieve sufficient consistency for the later handling not to cause excessive disorganization in the positioning of the fibers. Of course, the more advanced the prepolymerization and the better the fibers are maintained, but the more difficult it becomes to change the shape during later molding of the preform. Moreover, it is furthermore very difficult, or even virtually impossible, to interrupt this polymerization process, which sometimes may be very rapid owing to the exothermic nature of the reaction. In this case, the rigidity of the reinforced resin rapidly becomes too great, which is incompatible with later molding.
Thus, except for composite parts of planar or bar-shaped or rectilinear tube-shaped form, or for other simple forms, it has not hitherto been possible individually to position long fibers exactly in the desired orientation and in a correctly controlled density within the entire thickness of the part. It may be noted that, in the aforementioned patent, the fibers are cut to facilitate the working thereof. The result is unavoidable degradation of the reinforcement effect. “Fibers of great length” or “long fibers” or “fibers of infinite length” is understood to mean fibers, the length of which is limited only by the dimensions of the part, or at least by the dimensions of those sections of the part which have to be reinforced, without this length being limited by constraints resulting from the working process. “Individually positioning” means the fact of starting from spun yarns or simple flat fabrics, and not three-dimensional fabrics which are each time specific of a single manufactured part and which themselves also pose major handling problems.